1. Field of the Invention
This invention relates to a process for producing a photoelectric conversion device and to the photoelectric conversion device produced thereby. Particularly, the present invention relates to a process for forming a passivation film made of silicon oxide on a photoelectric conversion device having a junction between hydrogenated amorphous silicon and ITO (indium-tin oxide).
2. Description of the Prior Art
A photoelectric conversion device has been utilized as an image sensor for reading an original in facsimile equipment and the like.
FIG. 1 is a sectional view showing a conventional image sensor 1 constructed by the steps of forming a lower electrode 3 on an insulating substrate 2 so as to cover a part of the lower electrode 3, and providing a transparent upper electrode 5 on the hydrogenated amorphous silicon film 4.
Material for the insulating substrate 2 may be ceramics, glass, or plastics. Such insulating substrate must be of high mechanical strength and of high heat resistance so as not to be softened by heat which is applied to the substrate during manufacturing of the image sensor 1. In addition, high insulating properties are also required.
The lower electrode 3 is made of metal such as chromium (Cr), nickel (Ni), platinum (Pt), palladium (Pd), titanium (Ti), molybdenum (Mo), or tantalum (Ta), and is prepared, for example, by applying the metal in the form of a film having a thickness of 500 .ANG. to 5000 .ANG. onto the substrate 2. The metal coating is then subjected to photolithography and etching so as to be of suitable size and shape. As the hydrogenated amorphous silicon film 4, an undoped silicon film, a p-type silicon film doped with an element such as boron (B) or the like belonging to group III of the periodic table, or a like silicon film is employed. As a process for forming the amorphous silicon film 4, for example, the plasma CVD method has been adopted to deposit the film having a thickness of around 1 .mu.m.
For the transparent upper electrode 5, an ITO film is employed and the transparent upper electrode 5 is formed with a thickness of around 500 .ANG. to 2000 .ANG. in accordance with, for instance, a reactive deposition process, a reactive sputtering process or a like process.
The image sensor 1 having the above-mentioned laminated construction possesses excellent characteristic properties. For example, the ratio between light current and dark current (light/dark current ratio) is 10.sup.3 or more and the response time to a light pulse is 1 ms or less.
In this connection, the dark current is suppressed by means of an electron barrier layer (not shown) formed at the boundary of the amorphous silicon film 4 and the transparent upper electrode 5, and such an electron barrier layer is one of the factors determining the light/dark current ratio.
The image sensor 1 as described above has heat and humidity resistance which is not deteriorated even after annealing in air at 250.degree. C. for 1 hour under 1 atm of pressure, or even after auto-clave testing at 120.degree. C. for 30 minutes under 2 atm of pressure.
However, there has arisen problems of deterioration in the image sensor due to the staining of the transparent upper electrode 5 for receiving light and/or staining or abrasion by the mechanical contact of the image sensor 1 in the case where the image sensor is used in a picture information processor such as facsimile equipment or the like.
Furthermore, the image sensor 1 may decrease in its insulation properties when condensed water is produced on the image sensor.